1. Field of the Invention
The present invention relates to, in general, electronic circuitry for biasing signals. More specifically, the present invention relates to a circuit for pre-biasing the input pins of an integrated circuit during initialization of the integrated circuit.
2. Relevant Background
Integrated circuits having multiple pins for connections to external signals often require that these pins be externally biased to ensure proper operation of the integrated circuit. For example, the input pins for a CMOS device should be externally biased to a solid high or low logic level, away from the threshold regions, in order to prevent possible device oscillation and damaging supply-currents through the CMOS device.
Many integrated circuits also provide output signals connected to a bus of signals, such as an address bus or a data bus. These signal buses may be connected to multiple other devices or circuits. In order to preserve the integrity of the signal bus, any device connected to the signal bus should not access the bus (i.e., output a signal to the bus) at any improper time.
Integrated circuits conventionally have an output enable pin or control line which controls the drivers of the output pins of the integrated circuit. When the output enable control line is externally asserted, the output drivers of the integrated circuit are correspondingly enabled such that data appears on the output pins of the integrated circuit. When the output enable control line is disabled, the output pins of the integrated circuit are placed in a tri-state or high-impedance mode such that any signal line or bus to which the output pins are connected are unaffected thereby. In the high-impedance mode, the output pins of the integrated circuit essentially disconnect from any signal lines to which they are physically coupled.
For these reasons, the input pins of an integrated circuit should be biased and the output enable control line should be disabled while power is being applied to the integrated circuit. The biasing of the input pins and the output enable control line has traditionally been achieved through the use of components external to the integrated circuit. These components generally include pull-up or pull-down resistors and diodes which consume circuit board space, possibly conduct noise into the integrated circuit, and can add expense to the overall circuit design.
What is needed is an on-chip circuit for biasing the output enable control line and the other input pins of an integrated circuit during power-up conditions.